1. Field of the Invention
The present invention relates to a method of forming a thin copper film on the surface of a substrate by means of low temperature CVD.
2. Description of the Background
The technique of forming a thin copper film on the substrate surface is highly useful in various fields. For example, a thin copper film can be formed on the surface of a ceramic substrate by metallization. A ceramic substrate such as an alumina substrate is widely used as a packaging material of an IC or as a substrate for mounting an IC. When the ceramic substrate is used in such applications, it is necessary to form a wiring pattern on the substrate surface. For the wiring pattern formation, copper metallization is applied in general to the substrate surface so as to form a thin conductive film.
An alumina (Al.sub.2 O.sub.3) substrate is most widely used as a ceramic substrate. An alumina substrate exhibits excellent insulating properties, heat conductivity, resistance to chemicals, and mechanical strength, and is relatively cheap. On the other hand, marked progress is being made nowadays in the degree of integration, operating speed and output power of an IC, leading to a serious demand for a ceramic substrate with further improved heat-dissipating properties such as a substrate which is fully capable of acting as a heat sink. However, an alumina substrate normally has a heat conductivity of 10 to 35 W/mk and therefore fails to provide a sufficient heat sink function.
Beryllia (BeO) substrates and aluminum nitride (AlN) substrates are known as ceramic substrates which function fully as heat sinks. However, beryllia substrates, which have high heat conductivities ranging from 150 to 250 W/kb, are costly. In addition, BeO powder is poisonous which gives rise to difficulties in the manufacturing of beryllia substrates. On the other hand, aluminum nitride substrates have high heat conductivities on the order of 140 to 260 W/kb and have low manufacturing costs. In addition, AlN substrates exhibit excellent properties such as a low heat expansion coefficient, a high electrical insulation and a high mechanical strength. Thus, AlN substrates are expected to become more important in this field.
The technique of forming a thin copper film may also be applied to the coating of the surface of a stainless alloy plate with a thin copper film. Stainless alloys are already widely used. The application of a thin copper film coating is expected to further promote the use of stainless alloys. For example, the beautiful surface exhibited by a thin copper film coating makes stainless alloys suitable for use as a housing material, particularly, as an interior material and as an ornamental material. Stainless alloys exhibit poor solder wettability. However, solder wettability is markedly improved by the thin copper film coating, making it possible to use the stainless steel even in fields which require soldering. Further, the thin copper film coating eliminates the problem of stainless steel of having high electrical resistance. It follows that stainless steel coated with a thin copper film exhibits both the excellent spring properties of the stainless steel and the low electrical resistance of the copper film when the coated stainless steel is used in various applications such as in switches, terminals of a dry cell casing, and the like.
For forming a thin copper film on a substrate surface, various methods are known to the art including, for example, vacuum evaporation, sputtering, electroless plating, CVD and plasma CVD. However, it is necessary to heat the substrate to a temperature of 400.degree. to 1,300.degree. C. in each of these known methods except the electroless plating method and plasma CVD. Further, when it comes to the plasma CVD method, it is necessary to use a plasma generator which is very expensive. A need therefore continues to exist for an improved method of coating substrates with copper films.